1. Field of the Invention
This invention relates to an electrode used in an electrochemical cell such as a secondary battery and an electric double-layer capacitor and an electrochemical cell using the electrode. In particular, it relates to an electrode having improved cycle properties without reduction in an appearance capacity, and an electrochemical cell using the electrode.
2. Description of the Related Art
There have been suggested and practically used electrochemical cells (hereinafter, referred to as “cell”) such as secondary batteries and electric double-layer capacitors in which a proton-conducting compound is used as an electrode active material. Such a cell is illustrated in a cross-sectional view of FIG. 1.
Specifically, FIG. 1 shows a cell where a positive electrode 2 containing a proton-conducting compound as an active material is formed on a positive current collector 1 while a negative electrode 3 is formed on a negative current collector 4, and these electrodes are combined via a separator 5 and where only protons are involved in an electrode reaction as a charge carrier. Also, the cell is filled with an aqueous or non-aqueous solution containing a proton source as an electrolytic solution, and is sealed by a gasket 6.
The electrodes 2, 3 are formed as follows. A powdery doped or undoped proton-conducting compound is blended with a conductive auxiliary and a binder to prepare a slurry, which is then placed in a mold and molded by a hot press to form an electrode having a desired electrode density and a desired film thickness. Alternatively, the slurry is screen-printed on a conductive base-material and dried to form an electrode. Then, a positive electrode and a negative electrode thus formed are mutually faced via a separator to give a cell.
Examples of a proton-conducting compound used as an electrode active material include π-conjugated polymers such as polyaniline, polythiophene, polypyrrole, polyacetylene, poly-p-phenylene, polyphenylene-vinylene, polyperinaphthalene, polyfuran, polyflurane, polythienylene, polypyridinediyl, polyisothianaphthene, polyquinoxaline, polypyridine, polypyrimidine, polyindole, polyaminoanthraquinone and their derivatives; indole-based compounds such as indole trimer; and hydroxyl-containing polymers such as polyanthraquinone and polybenzoquinone where a quinone oxygen is converted into a hydroxyl group by conjugation). These compounds may be doped to form a redox pair exhibiting conductivity. These compounds are appropriately selected as a positive active material and a negative active material, taking a redox potential difference into account.
Known electrolytic solutions include an aqueous electrolytic solution consisting of an aqueous acid solution and a non-aqueous electrolytic solution based on an organic solvent. When using a proton-conducting compound, the former aqueous electrolytic solution is preferentially used because it can give a high-capacity cell. The acid used may be an organic or inorganic acid; for example, inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, tetrafluoroboric acid, hexafluorophosphoric acid and hexafluorosilicic acid and organic acids such as saturated monocarboxylic acids, aliphatic carboxylic acids, oxycarboxylic acids, p-toluenesulfonic acid, polyvinylsulfonic acid and lauric acid.
A cell using such a proton-conducting compound as an electrode active material has a short cycle life due to increase in an internal resistance, and the tendency becomes more prominent as a temperature is elevated. Furthermore, it has a drawback of insufficient long term stability under a high temperature atmosphere.
These problems are caused by aggravated deterioration atmosphere due to deceleration of proton adsorption-desorption reaction as a charge/discharge mechanism of an electrode active material. In particular, at an elevated temperature, peroxidation of a material is much more accelerated, resulting in accelerated deterioration.
An electrode active material is susceptible to deterioration in an oxidized state. It is probably because a proton (H+) adsorption-desorption reaction for the active material is deteriorated over time in the charge/discharge mechanism as described below. Such deterioration proceeds because doping/dedoping activity of the active material is reduced under an excess proton atmosphere rather than an optimal proton atmosphere which depends on the identity of the active material and the number of reaction electrons, in a proton adsorption-desorption reaction between the active material and an electrolyte. Thus, charge/discharge power of the cell is deteriorated. It is called “peroxidation-perreduction deterioration”; specifically, peroxidation deterioration for an active material of positive electrode and perreduction deterioration for an active material of negative electrode.
This phenomenon will be described for a case where an active material of positive electrode is an indole derivative (indole trimer) while an active material of negative electrode is a quinoxaline polymer. Herein, charge/discharge mechanisms for a positive and a negative electrode materials are as indicated in chemical formulas (8) and (9), respectively, wherein R represent appropriate substituents and X− represents an anion.

Under a high-level acid atmosphere (low pH), the phenomenon particularly tends to occur so that deterioration in cycle properties is accelerated. For polyphenylquinoxaline which can be used as a material of negative electrode, tetraprotonation may be caused whereas a normal doped state is represented by a diprotonated derivative in a charge/discharge mechanism. Thus, the active material is dissolved, leading to reduction in a charge/discharge power. An excessively higher electrolyte concentration (proton concentration) may further accelerate oxidation deterioration.
FIG. 6 is a graph showing variation in cycle properties to an electrolyte concentration (sulfuric acid concentration). As seen in this graph, as an electrolytic solution concentration increases, a capacity decreases according to the cycle number so that cycle properties are deteriorated. In addition, under a low concentration atmosphere, cycle properties are improved while an appearance capacity tends to be reduced. FIG. 7 is a graph illustrating variation in an appearance capacity to an electrolyte concentration (sulfuric acid concentration). As seen in this graph, as an electrolyte concentration is reduced, an appearance capacity is reduced.
Electrolytic solutions comprising a nitrogen-containing heterocyclic compound as a non-aqueous electrolytic solution in the prior art have been described in Japanese Laid-open Patent Publication Nos. 2000-156329 (Prior art 1) and 2001-143748 (Prior art 2). Japanese Laid-open Patent Publication No. 7-320780 (Prior art 3) has described a solid-electrolyte secondary battery comprising a polymer gel electrolyte consisting of, for example, an aprotic solvent and polyimidazole. Japanese Laid-open Patent Publication No. 10-321232 (Prior art 4) has described an electrode comprising a benzimidazole derivative although an electrolytic solution used therein is different from that in this invention.
In Prior art 1, there has been disclosed an electrolytic solution for an aluminum electrolysis capacitor comprising a quaternary salt having of a quaternary cation from a compound containing N,N,N′-substituted amidine group and an organic acid anion, and an organic solvent. There has been described that although a conventional electrolytic solution comprising a quaternary ammonium carboxylate has a drawback that degradation of a rubber packing is accelerated so that sealing performance is significantly deteriorated, an additive having a cationic, quaternary amidine group may improve thermal stability of the electrolytic solution and a specific conductivity, and that in particular, a compound in which electrons in the amidine group are delocalized and a cation is stabilized by resonance gives an improved specific conductivity because of accelerated ion dissociation. There has been further described that when excess hydroxide ions are generated after electrolysis in the electrolytic solution, the hydroxide ions may rapidly disappear by reaction of the hydroxide ions and the amidine group so that unlike a conventional quaternary ammonium salt, effects of the electrolysis can be reduced and thus degradation of a packing in a capacitor can be minimized, resulting in improved sealing performance.
Prior art 2 has disclosed an electrolytic solution for a non-aqueous electrolyte lithium secondary battery, comprising a lithium salt of a perfluoroalkylsulfonic acid dissolved in an organic solvent and at least one selected from heterocyclic compounds containing at least one fluorine atom and a nitrogen or oxygen atom. According to Prior art 2, the heterocyclic compound added to the electrolytic solution can form a strongly adsorptive and antioxidative film on a positive current collector, resulting in preventing oxidation deterioration of the positive current collector and thus improvement in cycle properties.
Prior art 3 has disclosed a solid electrolyte secondary battery comprising a positive electrode, a negative electrode containing lithium as an active material, and a polymer solid electrolyte consisting of a complex of an electrolyte salt with a polymer or a polymer gel electrolyte prepared by impregnating an electrolytic solution of an electrolyte salt dissolved in an aprotic solvent into a polymer, wherein the polymer is selected from the group consisting of a polyamide, polyimidazole, a polyimide, polyoxazole, polytetrafluoroethylene, polymelamineformamide, a polycarbonate and polypropylene. There is described that cycle properties are improved because the electrolyte is unreactive to the negative electrode and thus an internal resistance is unlikely to be increased even after repeating charge/discharge cycles.
For solving the problems of a reduced appearance capacity and deteriorated cycle properties seen in FIGS. 6 and 7, it is necessary to provide an optimal electrolyte composition (H+, X−), or to improve an electrode for preventing peroxidation-perreduction deterioration of an electrode active material in the reaction between an electrolyte and the active material.
In both Prior arts 1 and 2, a nitrogen-containing heterocyclic compound is added to a non-aqueous electrolytic solution. In Prior art 3, a polymer gel electrolyte consisting of, for example, an aprotic solvent and a polyimidazole is used to make the electrolyte unreactive to lithium in the negative electrode so that increase of an internal resistance can be minimized and thus cycle properties can be improved. In any of Prior arts 1, 2 and 3, a nitrogen-containing heterocyclic compound or its polymer is added to an electrolyte, which is different from this invention where a particular substance is added and blended in an electrode.
Since Prior art 4 relates to a lithium battery in which an electrolytic solution contains an organic solvent, a proton concentration is not taken into consideration. Thus, a mechanism of proton conductivity or deterioration as characteristics of an active material is considerably different. Prior art 4 is different from this invention in which an electrolytic solution contains a proton source and a proton-conducting compound is used as an active material.